An important property of high-pressure liquid chromatography (HPLC) separation media is that it must be highly and selectively retentive for the analyte of interest. Interest in carbon sorbents for HPLC is motivated by their ability to retain very polar analytes that would not be significantly retained or would not otherwise be separated on traditional C18-modified silica bonded phases. The unique selectivity originates from the fact that pure graphite is a crystalline material made up of sheets containing large numbers of hexagonally arranged sp2 hybridized carbon atoms linked by conjugated pi-bonds. These highly hydrophobic but exceptionally polarizable sheets of carbon have a high affinity for analytes that are also highly polarizable, highly polar or can accept a hydrogen bond. For example, polar nitrobenzene (π*dipolarity ˜1.11) is much more retained on carbon than is benzene (π*˜0.52). This is in contrast to conventional alkyl bonded phases that do not easily retain polarizable molecules.
There are many examples of applications that take advantage of the selectivity property including separation of water soluble micropollutants from water, drug metabolites, and pesticides. Additional applications include benzene and naphthalene sulfonates, acidic pesticides, and highly hydrophilic metabolites in blood and urine [10].Carbon also has advantages over synthetic polymeric supports that can shrink and swell as the solvent, pH, and ionic strength are changed. Other applications specific to carbon include selectivity for planar molecules. One example application for carbon sorption media is the trace extraction of coplanar polychlorinated biphenyls (PCBs), dibenzo-p-dioxins and dibenzofurans from other PCB congeners.
Particles useful as separation media in HPLC have many requirements that are met by most HPLC phases on the market today. This is in large part due to the fact that modern HPLC particles are based on porous silica. Commercially available silica particles for HPLC are typically stringently monodisperse and are mechanically strong enough to withstand packing pressures far exceeding 5000 psi. The particles are also spherical and are chemically stable in neutral and acidic solutions. Furthermore, the pore diameters of the silica are appropriately sized (60-350 Å) for liquid analysis and surface areas are high (>100 m2/g). Selectivity for analytes can be tuned to give supports whose surfaces have been bonded with various silanizing agents that impart a unique surface on the silica particle. The surface of the silica is often completely covered and is chemically homogeneous, as is well known in the art. However, complete surface coverage is not necessarily a requirement for desirable separations as is also well known in the art.
Carbon-coated sorption media has been widely employed for chromatographic support phase, including for HPLC phases. Carbon is an ideal adsorbent for a variety of analytes studied in HPLC, and therefore its use has been widely accepted.
The use of chemical vapor deposition (CVD) processes to produce carbon coatings has been extensively studied. Such processes are used, for example, to carbon coat nuclear materials or to infiltrate porous bodies so as to produce lightweight structural materials. This topic is discussed in more detail in The Chemistry and Physics of Carbon, 173-263 (P. Walker et al., eds. 1973), the disclosure of which is incorporated by reference herein.
Commercially available carbon-based supports useful for HPLC applications include carbon-clad zirconia/titanium and porous graphitic carbon (PGC). U.S. Pat. Nos. 5,254,262, and 5,108,597 describe the cladding of carbon on metal oxide particles such as zirconia. The inventors emphasize how to achieve monodispersity, extreme pH stability, and high efficiency. Using low pressure, high temperature vapor deposition, a carbon separation material was procured by pyrolizing a variety of saturated and unsaturated hydrocarbons on zirconia at 700° C. in a flow through horizontal rotary reactor. The residual exposed hard Lewis acid sites on the zirconia substrate causes irreversible adsorption of many Lewis bases. Thus, the adoption of this phase has been limited. Furthermore, the method and apparatus and process conditions as described in the patent has been found to be inadequate for obtaining coatings on silica particles.
In response to the fact that the process based on U.S. Pat. Nos. 5,254,262 and 5,108,597 is inadequate for obtaining significant carbon coatings on silica, Paek et al. described a process for obtaining carbon coated silica particles by first depositing metals (ex. Alumina) on the surface of the silica particles with subsequent carbonization by chemical vapor deposition using the method published in the aforementioned patents (Journal of Chromatography A Volume 1217, Issue 42, 15 Oct. 2010, Pages 6475-6483.) The Authors demonstrated that the coated materials could be packed into HPLC columns and that acceptable chromatographic performance could be achieved. This approach, however, also suffers from exposed Lewis acid sites, similar to that described above with respect to U.S. Pat. Nos. 5,254,262 and 5,108,597.
K. Unger et al. (U.S. Pat. No. 4,225,463) describe porous carbon support materials based on activated carbons and/or cokes. The materials are prepared by treating hard activated carbon or coke particles with solvents, and then heating them at 2400-3000° C. under an inert gas atmosphere. The resulting support materials are described as having a carbon content of at least 99 percent, a specific surface area of about 1-5 m2 per gram, and a particle size of about 5-50 μm. The resulting surface area and pore size are not generally reproducible. In addition, residual metals in the material adsorb analytes. It is generally understood that these materials are not useful for analytical separations.
U.S. Pat. No. 5,431,821 describes coating an oligomeric acetylene based polymer on the surface of porous silica with subsequent carbonization in a flow through batch reactor. The carbon source is deposited on the surface by evaporation and is subsequently carbonized. The make-up gas does not carry a carbon source. This material suffers from blocked pores, which limits its utility in HPLC.
Leboda et al. (Chromatographia, 13, 703 (1980); Chromatographia, 14, 524 (1981); Chromatographia, 13, 549 (1980)) describe chemical vapor deposited (CVD) methylene chloride on porous silica surfaces at 300-500° C. with subsequent deposition of a variety of aromatic alcohols (n-octanol) as a post treatment to complete the surface coverage. Leboda et al. used two methods to make the particles. First, Leboda et al. describe the two-hour pyrolysis of dichloromethane (CH2Cl2) on partially dehydroxylated silica gel (particle size range 0.15-0.30 mm) at 500° C. and atmospheric pressure in an autoclave. Additionally, Leboda et al., Chromatographia, 12, No. 4, 207-211 (1979) describe the catalytic decomposition of alcohol onto the surface of Si02 in an autoclave, at a pressure of 25 atmospheres and a temperature of 350° C. for 6 hours. The resulting material possesses a surface having from “a few to several dozen percent carbon on the surface.” The problem with this technique is that the particles are not mixed as the reaction occurs thus producing irregular coatings. Leboda et al. have also described a low pressure rotary reactor for the production of carbon coated particles to improve the mixing. Leboda et al. also describe seeding carbon deposition on the silica surface by depositing metals such as iron or alumina. This approach generally clogs the pores and gives poor chromatographic performance.
Leboda et al. further describe carbon-silica adsorbents obtained as a result of n-amyl alcohol pyrolysis. They concluded that these materials have a better ordered and more spatially developed structure of the carbon deposit than those obtained by dichloromethane pyrolysis. The carbon deposit formed by n-amyl alcohol pyrolysis is a better catalyst of carbonization than the deposit formed by dichloromethane pyrolysis.
Porous graphitic carbon is the most commercially successful HPLC carbon stationary phase on the market. It is prepared by filling the pores of a silica gel with a polymer comprising carbon, thermolyzing the polymer to produce a silica/carbon composite, dissolving out the silica to produce a porous carbon, and subjecting the porous carbon to graphitizing conditions. U.S. Pat. No. 4,203,268 describes a method for producing a porous carbon material suitable for chromatography or use as a catalyst support, which involves depositing carbon in the pores of a porous inorganic template material such as silica gel, porous glass, alumina or other porous refractory oxides having a surface area of at least 1 m2/g, and thereafter removing the template material. The resulting carbon is not a true graphite, but it does have a structure similar to two dimensional graphite making it chromatographically essentially indistinguishable from carbon that has been graphitized. Even though the material is mechanically stable enough to be used as a HPLC stationary phase, the material suffers from an expensive manufacturing process and is also 100% carbon, thus the phase is often prohibitively retentive. In addition, it has been reported that this phase is not stable at pressures exceeding 5000 psi and that the column life at those high pressures is short. Recent trends toward higher pressures (>5000 psi) have become a key trend in which is commonly employed in HPLC and it is clear that these particles will not withstand these ultra high pressures.
O. Chiantore et al., Analytical Chemistry, 60, 638-642 (1988), describe carbon sorbents which are prepared by pyrolysis of either phenol formaldehyde resin or saccharose on spheroidal silica gels coated with these materials. The pyrolysis is performed at 600° C. for one hour in an inert atmosphere, and the silica is subsequently removed by boiling the material in an excess of a 10% NaOH solution for 30 minutes.
Chiantore et al. conclude that, at the temperatures employed in their work, the carbonaceous polymer network that is formed still maintains the chemical features of the starting material. To obtain carbons where polar functional groups have been completely eliminated, the authors conclude that high temperatures (greater than 800° C.) treatments under inert atmosphere are necessary.
In addition to materials which comprise a carbon matrix or core, other HPLC chromatographic support materials are known which have a carbon coating on a substrate of silica. For example, N. K. Bebris et al., Chromatographia, 11, 206-211 (1978) describe the one-hour pyrolysis of benzene at 850° C. onto a substrate of Silochrom C-120, a macroporous silica (Si02) which contains particles of irregular form with an average size of 80 μm. Benzene pyrolysis was also carried out at 750° C. onto a substrate of Spherisorb S20W, a metals rich silica gel which contains spherical particles of diameter 20 μm. All of these efforts have been conducted on metals rich silica gel.
P. Carrott et al., Colloids and Surfaces, 12, 9-15 (1986) cracked furfuraldehyde vapor on precipitated silica at a temperature of 500° C. for various times, to achieve carbon loadings of 0.5, 8.6 and 16 percent. Carrott et al. conclude that the external surface of the resulting carbon-coated silica was hydrophobic, while the internal surface was hydrophilic, indicating that the internal surfaces are not well coated.
H. Colin et al., J. Chromatography, 149, 169-197 (1978) compare non-polar chemically-bonded phases (CBP), pyrocarbon-modified silica gel (PMS) and pyrocarbon-modified carbon black (PMCB) as packings for reversed phase HPLC. These phases all suffer from blocked pores and poor chromatographic performance.
Catalyst supports have also been prepared by deposition of carbon on alumina. For example, S. Butterworth et al., Applied Catalysis, 16, 375-388 (1985) describe a y-Al203 catalyst support having a coating of carbon deposited by vapor-phase pyrolysis of propylene. The pure phase y-Al203 substrate was ground to 12×37 mesh, had a bimodal pore size distribution based around mean diameters of 110 nm, and a surface area of 130 m2/g. When the vapor phase pyrolysis was performed from a flowing gas mixture of argon and propylene at 673 K, Butterworth et al. describe that the pure phase y-Al203 was completely covered at a carbon loading of 7 wt-%.
Certain metal oxides have been coated with carbon for use as nuclear reactor fuels. For example, P. Haas, Chemical Engineering Progress, 44-52 (April 1989) describes that small spheres of oxides of U, Th and Pu were required for high-temperature, gas-cooled nuclear reactor fuels. These fuels were coated with pyrolytic carbon or other ceramics to serve as “pressure vessels” which contain fission products.
FIG. 1 summarizes relevant prior art for carbon based particles that have been employed in HPLC. FIG. 1A shows a Type A silica that has been coated with pyrolytic carbon. In this case, either a metal is deliberately placed on the surface or is incorporated on the surface in various oxidation states and chemical forms. These sites act as nucleation sites for carbon formation on the surface. As is shown in the FIG. 1A, certain analytes can interact with the uncovered metals or with the uncovered Si—OH groups which become activated with an adjacent metal. One example of a common effect of these metal groups is increased retention of basic analytes under acidic eluent conditions. As previously noted, Paek et al. have deliberately placed metal on the substrate surface to aid carbon deposition. FIG. 1B shows a zirconia substrate that has been coated with a pyrolitic carbon. This material is not fully covered, so any Lewis base analytes may interact with the uncovered zirconia substrate and greatly increase retention or in some cases, prevent elution. It is a well known practice in the art to place a competing Lewis base into the eluent so as to compete with the analyte for the Lewis base sites on the chromatographic substrate. FIG. 1C. shows the surface of porous graphitic carbon. This material has been fully graphitized and is 100% carbon. It is well known that very polar analytes or planar species are highly retained on these materials. It is also clear that in contrast to other prior art materials, this material does not have any substrate. The absence of substrate has its advantages and disadvantages. It is well known that substrate-less materials are not useful for separations exceeding 400 bar as they are not typically mechanically stable or the packed bed is not stable at those hydrostatic pressures. It is significant to note that since the surface is 100% carbon, it can not be tuned and thus retention can not be tuned without unique and exotic mobile phase additives.
The particles of the present invention do not have the same problems as carbon coated on zirconia. These particles are free from residual Lewis Acid sites and will elute strong Lewis bases without a Lewis base eluent modifier such as phosphate. Even if enough coverage of the base zirconia is achieved so that an eluent modifier is not required, peak tailing associated with Lewis acid and Lewis base interaction will still be observed. In addition, the present invention teaches that in order to achieve a significant amount of carbon coating in a reasonable amount of time, one may employ increased pressure in the reactor to improve the level of carbon source present for reaction.
Our co-pending patent application number PCT/US 11/21537 describes a fluidized bed useful for obtaining carbon coated particles, and is incorporated herein by reference. The present invention is primarily directed to particles for HPLC, which are much smaller in diameter compared to solid phase extraction media. We have found that it is preferable to provide a fluidized bed reactor with a diameter of less than 6″ ID for HPLC particles. Advantages of such reactor size include better control of vapor deposition, better reproducibility of the fluid bed, and less loss of end product. In one embodiment, the fluidized bed diameter of the present invention is 3 inches. In yet another embodiment, the reactor diameter is 1 inch. Reactor lengths of 10-50 inches generate economically significant volumes.
Silica particles are known in the art to be classified as Type A or Type B, wherein Type B silica contains less than 20 ppm total metals, and Type A silica typically contains greater than 75 ppm total metals. For the purposes hereof, the term “metals” is intended to mean any element from the alkali, alkaline, transition, post transition, lanthanides, and actinide elemental groups, as is well understood in the art of chromatography. Common examples of metals found as impurities in silica may include sodium, calcium, iron, titanium, and zirconium, though it is also contemplated that any metal may be present as a contaminant in the particulate substrate. Metals are heat and electrical conductors, malleable, and can form cationic or ionic bonds with non-metals. Metals are considered as impurities that are detrimental to the goals of chromatographic separation due to the tendency to acidify the free silanol groups, and to render the separation generally less reproducible.
While carbon coatings on Type A silica have been described as a result of the beneficial metal nucleation sites available in Type A silica, efforts to carbon coat Type B silica have heretofore been unsuccessful. It has been found that Type B silica may be advantageous as an HPLC support because it is ultra pure and is virtually metals free. Metals that are incorporated into the silica backbone or are bonded or are adsorbed on the surface can cause a variety of deleterious effects on chromatography as is commonly described in the art. The metals found in Type A silica can be elemental, oxides, ions, or any other metal form and include but are not limited to zirconia, titanium, iron, aluminum, magnesium, sodium, calcium, potassium, chromium, copper, and zinc. High levels of residual metals in Type A silica cause increased retention of amines and generally result in less reproducible chromatographic elutions.
Using the particles, method, and apparatus of the present invention, we have discovered that carbon may be successfully coated on substantially metal-free inorganic oxide substrates, such as Type B silica. In addition, we have found that desirable carbon loading can be achieved on such substrates that are fully or partially hydroxylated. In a preferred embodiment, the substrate of the present invention has a total metals content of less than 50 ppm. However, it is anticipated that a Type A silica could be carbonized with the method of the present invention and then post treated with an acid to reduce the total metals to a desired level such as less than 50 ppm.
Conventional coating techniques fail to yield a chromatographically significant coating of carbon on low-metal substrates unless the surface has been pre-treated with a metal as disclosed by Paek et al. The present invention introduces a method for obtaining carbon coatings on substantially metal-free substrates, such as Type B, silica to form a desirable chromatographic support media.
Furthermore, the present coating technique is applicable to a multiplicity of low or no metal substrates. The present invention is effective in coating particles of various diameters and pore sizes. Using the methods of the present invention, carbon may also be coated on superficially porous particles to provide for ultra-fast separations.
Another issue in the manufacture of chromatographic-grade carbon-coated silica is that the small diameter silica particles (<10 μm) tend to cling to the walls of the reactor. Carbon-coated zirconia or large (>30 μm diameter) silica particles do not pose such problem because, unlike small silica particles, these particles are free flowing and much more dense. Particle caking and static cling of small diameter silica particles results in agglomeration in the reactor and inhomogeneous reaction, thus causing irreproducible retention times for select analytes in chromatographic analysis. We have also found that small particles are more likely to be carried by the carrier gas out of the reactor zone. If a horizontal rotary reactor is used, upon completion of the reaction, uncoated particles that have been carried out of the reactor zone are inevitably mixed with the coated particles. The present invention overcomes this problem by implementing a fluidized bed reactor with a specific filter at the outlet of the reactor.
In addition to the above, the prior art is silent regarding the utility of tuning or pre-selecting the amount of carbon deposited on the substrate for sorbent and separation applications. The present invention provides the selection of various amounts of carbon deposited on various types and sizes of particles. Thus, the retention time of a selected analyte may be pre-determined by the extent of the carbon loading applied to the substrate